Pulmonary hypertension, defined as mean PA (pulmonary arterial) pressure of > 25 mm Hg, is an end point of a variety of conditions. These include primary pulmonary hypertension, post-operative pulmonary hypertension and as a result of increased pulmonary vascular resistance that occurs in ARDS. This is an entity distinguished by hypoxemia, non-cardiogenic pulmonary edema and bilateral infiltrates on chest X-ray. Prostacyclin (PGE1), an arachidonic acid metabolite, or its stable analogue, iloprost, has been evaluated for its efficacy in the treatment of pulmonary hypertension and for its use in reducing intra-pulmonary shunting in ARDS. While the intravenous application of iloprost can cause a decrease not only of the pulmonary but also of the systemic vascular tone, the aerosolized prostacyclin can result in a selective pulmonary vasodilatation without affecting the systemic blood pressure as demonstrated in preliminary studies/reports. Furthermore, aerosolized iloprost can improve gas exchange and pulmonary shunt in clinical settings of impaired ventilation/perfusion ratio as it occurs in ARDS, due to the redistribution of pulmonary blood flow from non-ventilated to ventilated, aerosol accessible lung regions.